1. Field of the Invention
The invention relates to the field of hearing aids. The invention, more specifically, relates to a hearing aid having an adaptive filter for suppressing acoustic feedback. The invention also relates to a method of adaptively reducing acoustic feedback of a hearing aid and to an electronic circuit for a hearing aid.
2. Description of the Related Art
Acoustic feedback occurs in all hearing instruments when sounds leak from the vent or seal between the earmould and the ear canal. In most cases, acoustic feedback is not audible. But when the in-situ gain of the hearing aid is sufficiently high, or when a larger than optimal size vent is used, the gain of the hearing aid can exceed the attenuation offered by the ear mould/shell. The output of the hearing aid then becomes unstable and the once-inaudible acoustic feedback becomes audible, e.g. in the form of a whistling noise. For many users and people around, such audible acoustic feedback is an annoyance and even an embarrassment. In addition, hearing instruments that are at the verge of feedback, i.e. in a state of sub-oscillatory feedback, may suffer an adverse influence to the frequency characteristic of the hearing instrument, and potentially intermittent whistling.
Generally a hearing aid comprises an input transducer or microphone transforming an acoustic input signal, a signal processor amplifying the input signal and generating an electrical output signal and an output transducer or receiver for transforming the electrical output signal into an acoustic output. The acoustic propagation path from the output transducer to the input transducer is referred to as the acoustic feedback path of the hearing aid, the attenuation factor of the feedback path being denoted by β. If, in a certain frequency range, the product of gain G (including transformation efficiency of microphone and receiver) of the processor and the attenuation β is close to 1, audible acoustic feedback occurs.
WO-A1-02/25996 describes a hearing aid including an adaptive filter intended to suppress undesired feedback. The adaptive filter estimates the transfer function from output to input of the hearing aid including the acoustic propagation path from the output transducer to the input transducer. The input of the adaptive filter is connected to the output of the hearing aid and the output signal of the adaptive feedback estimation filter is subtracted from the input transducer signal to compensate for the acoustic feedback. In this hearing aid the output signal from the signal processor is fed to an adaptive feedback estimation filter, which is controlled by a filter control unit. The adaptive feedback estimation filter constantly monitors the feedback path providing an estimate of the feedback signal and producing an output signal which is subtracted from the processor input signal in order to reduce, or in the ideal case to eliminate, acoustic feedback in the signal path of the hearing aid.
An overview of adaptive filtering is given in the textbook of Philipp A. Regalia: “Adaptive IIR Filtering in Signal Processing and Control”, published in 1995.
One problem associated with adaptive feedback cancelling is a bias introduced by the feedback prediction model itself through narrow band signals included e.g. in speech or music. The correlation analysis of the adaptive feedback estimation algorithm is based on the assumption that a feedback signal (oscillation) is a highly correlated version of the original signal. When signal components of the external hearing aid input, e.g. contained in speech or music, are narrow band signals, a bias is introduced in the feedback prediction model and the external narrow band signal components are removed from the hearing aid signal path by the feedback suppression algorithm.
Siqueira and Alwan propose, in “Steady-State Analysis of Continuous Adaptation in Acoustic Feedback Reduction Systems for Hearing Aids”, IEEE transactions on speech and audio processing, Vol. XIII, no. 4, pages 443-453, July 2000, the use of a delay in the forward or cancellation path of the hearing aid in order to reduce the bias introduced by narrow band input signals. This delay, however, does still not make a sinosoid signal unpredictable by the feedback cancellation algorithm.
US 2003/0053647 A1 to Kates shows a hearing aid comprising a cascade of adaptive notch filters for processing to the error signal before a signal is supplied to the feedback path estimation algorithm. The series of notch filters removes the narrow band signal components from the feedback estimation algorithm so that the mean square error (MSE) calculation in the adaptive feedback estimation filter does not take into account the external narrow band signal components and interpolates the feedback path model over the absent frequencies.
To ensure a correct mean square error minimization process with respect to the narrow band filtered error signal the input signal of the adaptive feedback estimation filter must be filtered with copies of the adaptive notch filters before it is fed to the adaptation algorithm.
Furthermore, the narrow band filters are optimized to cancel the narrow band signal components by minimizing a cost function of the narrow band filter output.
In order to remove a plurality of narrow band signal components a plurality of notch filters are required. With an increasing number of notch filters for different frequencies, however, the computational costs increase and mutual influence of the different notch filters may occur.